1. Technical Field
The invention relates to the field of inkjet printing. More specifically the invention relates to systems for automatically bleeding air from an ink delivery system.
2. Description of the Related Art
Inkjet printing involves depositing droplets of liquid ink onto a printing medium from one or more printer heads. The printer heads are coupled with a container containing ink. Ink is ejected from one or more nozzles of the print heads when a piezoelectric crystal in the print head is actuated. The piezoelectric crystal generates a pulse in the ink so that the ink expels through the nozzle as a droplet. To create the image, a carriage which holds one or more print heads scans or traverses across the printing medium, while the print heads deposit ink as the printing medium moves.
Small desktop inkjet printers are common consumer electronic products. Indeed, many consumer and business printing needs may be met by small desktop inkjet printing systems because of the relatively small amount of ink needed for common print jobs. However, some printing applications require much larger amounts of ink. For instance, large format printing is performed to create signs, banners, museum displays, sails, bus boards and the like. These types of applications require large throughput printers and require a much larger quantity of ink.
Ink cartridges are typically sold with replaceable ink reservoirs. Most commonly, these ink reservoirs are individually packaged and sold over the counter. However, common inkjet reservoirs contain far less ink than is required for large format printing. Currently, replacement reservoirs are not available in volumes greater than approximately five liters. Furthermore, the overhead cost associated with individually manufacturing, packaging and shipping small, individual replacement reservoirs is burdensome given that they must be replaced frequently to achieve large format printing. Accordingly, many print applications benefit from bulk ink supply systems.
Typical bulk ink supply systems for inkjet printers involve supplying the print head of the inkjet printer with ink from a bulk reservoir remote from the print head via ink lines. Some approaches in bulk ink supply involve a gravity feed, capillary feed, siphons or other mechanisms, instead of active electrical/mechanical devices, to transfer ink to the printing head. However, gravity feed ink delivery systems have inherent limitations, as their use often results in ink starvation or flooding at the printing head. These phenomena occur because the level of the ink immediately adjacent to the printing head is insufficiently maintained either due to limitations of the feed system or the need to manually adjust and replenish the ink reserves.
Other approaches to bulk ink delivery system involve a pump configured to suck ink from the bulk reservoir through a filter to the print heads via supply lines. However, sucking ink through a filter creates microbubbles that are mixed into the ink. Another drawback to this conventional approach using a pump system is that once a reservoir is run dry the pump begins to pump air into the supply lines. Additionally, even if the reservoir is changed before it become dry, the supply lines become de-primed when an operator changes the bulk reservoir, thereby introducing air into the system.
Indeed, the presence of air in an inject system is problematic. For example, if air is present in the ink chamber within the print head, intended pressure changes resulting from piezoelectric deformation of part of the ink chamber walls will be absorbed by the air, leaving the ink pressure unaffected. The surface tension force of the ink in the nozzle maintains the meniscus and fewer or no drops will be ejected from the ink chamber or the drops will be misdirected.
Previous attempts to limit the presence of ink involve the use of an air trap, utilizing a float to shut the air exit off once the air is removed. However, the air trap has moving parts, is expensive, bulky and is not always reliable.
Accordingly, there is a need for a system of using bulk ink reservoirs to supply an inkjet system in which air is not mixed with the ink that is pumped to the print heads which is reliable and commercially feasible.